Would a Shambling Mound let a Will-o’-Wisp hit it to absorb the Lightning dmg

@ChrisPerkinsDnD would a shambling mound let a will o wisp hit it to absorb the Lightning dmg or would the wisp have to make a atk roll?

— Wait…What? (@davoerlo) November 7, 2016

The mound might be attracted to the wisp and consider it an ally of sorts. No attack roll req'd. #WOTCstaff https://t.co/XLzSDxbkwe

— Christopher Perkins (@ChrisPerkinsDnD) November 7, 2016